There are only two client interfaces included in the base
PostgreSQL distribution:

libpq is included because it is
the primary C language interface, and because many other
client interfaces are built on top of it.

ECPG is included because it
depends on the server-side SQL grammar, and is therefore
sensitive to changes in PostgreSQL itself.

All other language interfaces are external projects and are
distributed separately. Table H-1
includes a list of some of these projects. Note that some of
these packages might not be released under the same license as
PostgreSQL. For more information
on each language interface, including licensing terms, refer to
its website and documentation.